Real-World Constraints on Global Warming - Fraser … · 91 Real-World Constraints on Global Warming SHERWOOD B. IDSO Often lost in the heat of debate over global warming is the fact

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Real-World Constraints on Global Warming

SHERWOOD B. IDSO

Often lost in the heat of debate over global warming is the factthat the greenhouse effect of CO2 is not in question: rising car-bon dioxide concentrations, in and of themselves, do indeedhave a tendency to enhance the thermal-blanketing properties ofthe atmosphere. Opinions diverge, however, about what hap-pens when this phenomenon begins to operate, for practicallynothing in nature happens in isolation, and numerous feed-backs—both positive and negative—severely cloud the issue.

Proponents of the theory that a dangerous level ofCO2-induced global warming is likely to occur believe that pos-itive feedbacks that enhance the initial rise in temperature pro-duce the major portion of whatever temperature increaseultimately occurs (or is predicted to occur). Detractors of thetheory that a dangerous level of CO2-induced global warming islikely to occur believe that negative feedbacks are capable ofdrastically reducing the initial impetus for warming. Some, likemyself, believe that such feedbacks can negate totally the initialwarming impetus; and it is the evidence for this latter positionthat I will discuss in this small treatise.

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Negative feedbacksThere are at least three major categories of negative feedbackmechanisms that are germane to the issue of CO2-induced glo-bal warming. First—and most recognized—are the mechanismsby which rising temperatures may strengthen the cooling prop-erties of clouds by purely physical means. Second are the mech-anisms by which rising temperatures intensify biologicalprocesses that eventually lead to an enhancement of some of thesame cloud-cooling properties. Third are the mechanisms bywhich some of these biological processes are directly enhancedby the “aerial fertilization effect” of increased concentrations ofatmospheric CO2 so that they are not dependent upon an initialwarming to set the ultimate cloud-cooling processes in motion.In the following subsections, I shall describe these three types ofnegative feedbacks briefly and give illustrations of their greatcooling power.

Feedbacks using physical processesIt has long been recognized that the presence of clouds has astrong cooling effect on earth’s climate (Barkstrom 1984; ERBEScience Team 1986; Nullet 1987; Nullet and Ekern 1988; Ra-manathan et al. 1989). In fact, it has been calculated that a mereone percent increase in planetary albedo (i.e. the ratio of lightreflected by the earth to that received by it) would be sufficientto counter totally the entire greenhouse warming that is typical-ly predicted to result from a doubling of the atmosphere’s CO2concentration (Ramanathan 1988).

Within this context, it has been independently demonstratedthat a 10 percent increase in the amount of low-level cloud couldalso completely cancel the warming that is typically predicted tooccur as a result of a doubling of the air’s CO2 content, again byreflecting more solar radiation back to space (Webster andStephens 1984). In addition, Ramanathan and Collins (1991),by the use of certain “natural experiments,” have shown how thewarming-induced production of high-level clouds over the equa-torial oceans almost totally nullifies the powerful greenhouse ef-fect of water vapor there. In fact, Kiehl (1994) has describedhow the presence of high-level clouds in this area dramaticallyincreases from close to 0 percent coverage when temperatures atthe sea’s surface are 26°C to fully 30 percent coverage when theyare 29°C. The implication of this strong negative feedback mech-

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anism is that “it would take more than an order-of-magnitudeincrease in atmospheric CO2 to increase the maximum sea sur-face temperature by a few degrees” (Ramanathan and Collins1991: 32). They acknowledge that this estimate is a considerabledeparture from the predictions of most general circulation mod-els of the atmosphere but I will shortly show it to be in completeharmony with a variety of real-world observations.

In addition to increasing their areal coverage of the planet—asthey typically do in response to an increase in temperature(Henderson-Sellers 1986a, 1986b; McGuffie and Henderson-Sellers 1988; Dai et al. 1997)—clouds in a warmer world wouldlikely have greater liquid-water content than they do now (Pal-tridge 1980; Charlock 1981, 1982; Roeckner 1988). And, as theheat-conserving greenhouse properties of low- to mid-levelclouds are already close to the maximum they can attain (Bettsand Harshvardhan 1987) while their reflectances for solar radi-ation may yet rise substantially (Roeckner et al. 1987), an in-crease in the liquid-water content of clouds would tend tocounteract an impetus for warming even in the absence of an in-crease in cloud cover. In fact, by incorporating just this one neg-ative feedback mechanism into a radiative-convective climatemodel, the warming predicted to result from a doubling of theair’s CO2 content has been shown to fall by fully 50 percent(Somerville and Remer 1984) while a 20 percent to 25 percentincrease in liquid water in clouds has been shown, in a three-di-mensional general circulation model of the atmosphere, to ne-gate totally the typically predicted warming due to a doubling ofthe air’s CO2 content (Slingo 1990).

Feedbacks using biological processesCharlson et al. (1987) have described another negative feedbackmechanism involving clouds, which has been calculated to be ofthe same strength as the typically predicted greenhouse effect ofCO2 (Lovelock 1988; Turner et al. 1996). These investigatorssuggest that the productivity of oceanic phytoplankton will in-crease in response to an initial impetus for warming, with the re-sult that one of the ultimate by-products of the enhanced algalmetabolism—dimethyl sulfide (DMS)—will be produced in sig-nificantly greater quantities. Diffusing into the atmosphere,where it is oxidized and converted into particles that function ascloud-condensation nuclei, this augmented flux of DMS has

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been projected to create additional clouds and/or clouds with ahigher albedo. This should reflect more solar radiation back tospace, thereby cooling the earth and countering the initial impe-tus for warming (Shaw 1983, 1987).

There is much evidence—700 papers in the past 10 years(Andreae and Crutzen 1997)—to support the validity of eachlink in this conceptual chain of events. First, there is the dem-onstrated propensity for oceanic phytoplankton to increasetheir productivity in response to an increase in temperature(Eppley 1972; Goldman and Carpenter 1974; Rhea and Goth-am 1981); this propensity is clearly evident in latitudinal dis-tributions of marine productivity (Platt and Sathyendranath1988; Sakshaug 1988). Second, as oceanic phytoplankton pho-tosynthesize, they produce a substance called dimethylsulfoniopropionate (Vairavamurthy et al. 1985), which dispersesthroughout the surface waters of the oceans when the phy-toplankton die or are eaten by zooplankton (Nguyen et al.1988; Dacey and Wakeham 1988) and decomposes to produceDMS (Turner et al. 1988). Third, it has been shown that part ofthe DMS thus released to the earth’s oceans diffuses into theatmosphere, where it is oxidized and converted into sulfuricand methanesulfonic acid particles (Bonsang et al. 1980;Hatakeyama et al. 1982; Saltzman et al. 1983; Andreae et al.1988; Kreidenweis and Seinfeld 1988) that function as cloud-condensation nuclei (CCN) (Saxena 1983; Bates et al. 1987).Fourth, more CCN can clearly stimulate the production of newclouds and dramatically increase the albedos of pre- existentclouds by decreasing the sizes of the clouds’ component drop-lets (Twomey and Warner 1967; Warner and Twomey 1967;Hudson 1983; Coakley et al. 1987; Charlson and Bates 1988;Durkee 1988). This latter phenomenon then tends to cool theplanet by enabling clouds to reflect more solar radiation backto space (Idso 1992b; Saxena et al. 1996). In fact, it has beencalculated that a 15 percent to 20 percent reduction in themean droplet radius of earth’s boundary-layer clouds wouldproduce a cooling influence that could completely cancel thetypically predicted warming due to a doubling of the air’s CO2content (Slingo 1990).

Another way in which the enhanced production of CCN mayretard global warming via a decrease in size of droplets in cloudsis by reducing drizzle from low-level marine clouds, which length-


ens their life span and thereby expands their coverage of the plan-et (Albrecht 1988). In addition, since drizzle from stratus cloudstends to stabilize the atmospheric boundary layer by cooling thesub-cloud layer as a portion of the drizzle evaporates (Brost et al.1982; Nicholls 1984), a CCN-induced reduction in drizzle tendsto weaken the stable stratification of the boundary layer, enhanc-ing the transport of water vapor from ocean to cloud. As a result,clouds containing extra CCN tend to persist longer and performtheir cooling function for a longer period of time.

The greater numbers of CCNs needed to enhance these sev-eral cooling phenomena are also produced by biological pro-cesses on land (Went 1966; Duce et al. 1983; Roosen andAngione 1984; Meszaros 1988) and in the terrestrial environ-ment, the volatilization of reduced-sulfur gases from soils isparticularly important in this regard (Idso 1990). Here, too, oneof the ways in which the ultimate cooling effect is set in motionis by an initial impetus for warming. It has been reported, forexample, that soil DMS emissions rise by a factor of two foreach 5°C increase in temperature between 10°C and 25°C(Staubes et al. 1989) and, as a result of the enhanced microbialactivity produced by increasing warmth (Hill et al. 1978; Mac-Taggart et al. 1987), there is a 25-fold increase in soil-to-air sul-fur flux between 25°N and the equator (Adams et al. 1981). Ofperhaps even greater importance, however, is the fact that in-creased concentrations of atmospheric CO2 alone can initiatethe chain of events that leads to cooling.

Feedbacks resulting from increased levels of atmospheric CO2Consider the fact—impressively supported by literally hundredsof laboratory and field experiments (Lemon 1983; Cure andAcock 1986; Mortensen 1987; Lawlor and Mitchell 1991; Drake1992; Poorter 1993; Idso and Idso 1994; Strain and Cure1994)—that nearly all plants are better adapted to concentra-tions of atmospheric CO2 higher than those of the present, andthat the productivity of most herbaceous plants rises by 30 per-cent to 40 percent in the presence of a 300 ppm to 600 ppm in-crease in the air’s CO2 content (Kimball 1983; Idso 1992a),while the growth of many woody plants rises even more dramat-ically (Idso and Kimball 1993; Ceulemans and Mousseau 1994;Wullschleger et al. 1995, 1997). Because of this stimulatory

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effect on plant growth and development, the productivity of thebiosphere has been rising hand in hand with the recent rise inthe air’s CO2 content (Idso 1995); this is evident in (1) the everincreasing amplitude of the seasonal cycle of the air’s CO2 con-centration (Pearman and Hyson 1981; Cleveland et al. 1983; Ba-castow et al. 1985; Keeling et al. 1985, 1995, 1996; Myneni et al.1997), (2) the upward trends in a number of long tree-ringrecords that mirror the progression of the Industrial Revolution(LaMarche et al. 1984; Graybill and Idso 1993; Idso 1995), and(3) the accelerating growth rates of numerous forests on nearlyevery continent of the globe over the past several decades(Kauppi et al. 1992; Phillips and Gentry 1994; Pimm and Sugden1994; Idso 1995).

In consequence of this CO2-induced increase in plant produc-tivity, more organic matter is returned to the soil (Leavitt et al.1994; Jongen et al. 1995; Batjes and Sombroek 1997), where itstimulates biological activity (Curtis et al. 1990; Zak et al. 1993;O’Neill 1994; Rogers et al. 1994; Ineichen et al. 1997; Ringel-berg et al. 1997; Godbold and Berntson 1997) that results in theenhanced emission of various sulfur gases to the atmosphere(Staubes et al. 1989), whereupon more CCNs are created (as de-scribed above), which tend to cool the planet by altering cloudproperties in ways that result in the reflection of more solar ra-diation back to space. In addition, many non-sulfur biogenicmaterials of the terrestrial environment play major roles asboth water- and ice-nucleating aerosols (Schnell and Vali 1976;Vali et al. 1976; Bigg 1990; Novakov and Penner 1993; Saxena etal. 1995; Baker 1997); and the airborne presence of these mate-rials should also be enhanced by increased concentrations of at-mospheric CO2.

Analogous CO2-induced cooling processes likely operate atsea as well. It is well established, for example, that increasedconcentrations of atmospheric CO2 stimulates the growth ofboth macro-aquatic plants (Titus et al. 1990; Sand-Jensen et al.1992; Titus 1992; Madsen 1993; Madsen and Sand-Jensen 1994)and micro-aquatic plants (Raven 1991, 1993; Riebesell 1993;Shapiro 1997). In addition, it has been demonstrated in a majorexperimental program (Coale et al. 1996) that adding iron to thehigh-nitrate low-chlorophyll waters of the equatorial Pacific sig-nificantly stimulates the productivity of oceanic phytoplankton(Behrenfeld et al. 1996) and this surrogate for a CO2-induced in-


crease in marine productivity has been observed to increasesurface-water DMS concentrations greatly (Turner et al. 1996).There is also evidence to suggest that a significant fraction of theice-forming nuclei of maritime origin are composed of organicmatter (Rosinski et al. 1986, 1987); and the distribution of thesenuclei over the oceans (Bigg 1973) has been shown to be strong-ly correlated with surface patterns of biological productivity(Bigg 1996; Szyrmer and Zawadzki 1997). Hence, it is clear thatthere exists an entire suite of powerful planetary cooling forcesthat can respond directly to the rising carbon dioxide content ofthe atmosphere over both land and sea. And these CO2-inducedcooling forces could negate a large portion (or even all) of theprimary warming effect of a rise in atmospheric CO2, leading tolittle or no net change in mean global air temperature.

Evidence for muted global warmingThe power of nature’s negative feedbacks, like the theory ofCO2-induced global warming, must be evaluated againstreal-world evidence. Hence, in the subsections that follow, Imake such evaluations for 4 global climatic situations that incor-porate all the real-world phenomena that combine to producethe equilibrium results derived therein.

The greenhouse effect in the earth’s whole atmosphereThe current greenhouse effect of earth’s entire atmospherewarms the surface of the planet by approximately 33.6°C as theresult of a surface-directed thermal radiation flux of approxi-mately 348Wm–2 (Watts per square metre) (Idso 1980, 1982).Dividing the first of these numbers by the second yields whatcould be called a surface air-temperature sensitivity factor,which for this particular situation has a value of 0.097°C/Wm–2.Multiplying this factor by 4Wm–2—the value by which the fluxof thermal radiation to the earth’s surface is expected to rise asa result of a 300 ppm to 600 ppm increase in the air’s CO2 con-centration (Smagorinsky et al. 1982; Nierenberg et al. 1983;Shine et al. 1990)—yields a mean global warming of 0.39°C,which is but one-tenth to one-third of the warming that hasbeen predicted for this scenario by the majority of the general-circulation models of the atmosphere that have been applied tothis problem (Kacholia and Reck 1997).

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Latitudinally-dependent greenhouse effectA second evaluation of the likely warming to be expected from adoubling of the air’s CO2 content can be derived from the annu-ally averaged equator-to-pole air-temperature gradient that issustained by the annually averaged equator-to-pole gradient oftotal radiant energy absorbed at the surface (Idso 1984). Meansurface air-temperatures and water-vapor pressures required forthis calculation can be obtained for each five-degree latitude in-crement stretching from 90°N to 90°S from information reportedby Warren and Schneider (1979) and Haurwitz and Austin(1944). From these data, I calculated values of clear-sky atmo-spheric thermal radiation (Idso 1981) incident upon the surfaceof the earth at the midpoints of each of the specified latitudebelts. Then, from information about the latitudinal distributionof cloud cover (Sellers 1965) and the ways in which clouds mod-ify the clear-sky flux of downwelling thermal radiation at theearth’s surface (Kimball et al. 1982), I appropriately modified theclear-sky thermal radiation fluxes and averaged the results overboth hemispheres. Similarly averaged fluxes of surface-absorbedsolar radiation (Sellers 1965) were then added to the thermal-ra-diation results to produce 19 annually averaged total surface-ab-sorbed radiant-energy fluxes stretching from the equator to90°N/S, against which I plotted the corresponding average valuesof mean surface air temperature.

This operation produced two distinct linear relationships—one of slope 0.196°C/Wm–2, which extended from 90°N/S to ap-proximately 63°N/S, and one of slope 0.090°C/Wm–2, which ex-tended from 63°N/S to the equator. I thus weighted the tworesults according to the percentages of earth’s surface area towhich they pertained (12 percent and 88 percent, respectively)and combined them to obtain a mean global value of 0.103°C/Wm–2. Multiplying this result, as before, by 4Wm–2 then yieldsa mean global warming of approximately 0.41°C, which is essen-tially the same amount of warming I derived from the priorwhole-atmosphere calculation.

The greenhouse effect from an increased concentration of atmospheric CO2 over geologic time The same result may also be obtained from the standard resolu-tion of the paradox of the faint early sun (Sagan and Mullen1972; Owen et al. 1979; Kasting 1997), a dilemma (Sagan and


Chyba 1997; Longdoz and Francois 1997) that is most oftenposed by the following question: how could earth have support-ed life nearly 4 billion years ago when, according to well-estab-lished concepts of stellar evolution (Schwarzchild et al. 1957;Ezer and Cameron 1965; Bahcall and Shaviv 1968; Iben 1969),the luminosity of the sun was probably 20 percent to 30 percentless than it is now (Newman and Rood 1977; Gough 1981), sothat, all else being equal, nearly all of earth’s water should havebeen frozen and unavailable for sustaining life (Schopf and Barg-hourn 1967; Knauth and Epstein 1976; Schopf 1978; Lowe1980; Schidlowski 1988)?

Most who have studied the problem feel that the answer tothis question resides primarily in the large greenhouse effect ofearth’s early atmosphere, which is believed to have containedmuch more CO2 than it does today (Hart 1978; Holland 1984;Wigley and Brimblecombe 1981; Walker 1985). Consequently,based on the standard assumption of a 25 percent reduction insolar luminosity 4.5 billion years ago, I calculated the strengthof the CO2 greenhouse effect required to compensate for the ef-fects of reduced solar luminosity at half-billion year intervalsfrom 3.5 billion years ago—when we are confident of the wide-spread existence of life (Mojzsis et al. 1996; Eiler et al. 1997)—to the present. I plotted the results as a function of the atmo-spheric CO2 concentration derived from a widely accepted atmo-spheric CO2 history for that period of time (Lovelock andWhitfield 1982). Using the relationship derived from that exer-cise to calculate the effects of a 300 ppm to 600 ppm increase inthe air’s CO2 concentration, I once again obtained a mean globalwarming of only 0.4°C (Idso 1988).

The greenhouse effect from increased concentration of atmospheric CO2 on Mars and Venus Consider, finally, what we can learn from our nearest planetaryneighbors, Mars and Venus. In spite of the tremendous differ-ences that exist between them, and between them and the earth,their observed surface temperatures have been said to confirm“the existence, nature, and magnitude of the greenhouse effect”((Smagorinsky et al. 1982: 5; Nierenberg et al. 1983: 274) by twoselect committees of the United States National Research Coun-cil, a conclusion that also appears to be accepted by the Intergov-ernmental Panel on Climate Change (Trenberth et al. 1996).

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Venus exhibits a greenhouse warming of approximately 500°C(Oyama et al. 1979; Pollack et al. 1980) that is produced by a93-bar atmosphere of approximately 96 percent CO2 (Kasting etal. 1988); Mars exhibits a greenhouse warming of 5 to 6°C (Pol-lack 1979; Kasting et al. 1988) that is produced by an almostpure CO2 atmosphere that fluctuates over the Martian year be-tween 0.007 and 0.010 bar (McKay 1983). Plotting the twopoints defined by these data on a log-log coordinate system ofCO2-induced global warming versus the partial pressure of at-mospheric CO2 and connecting them by a straight line producesa relationship that, when extrapolated to CO2 partial pressurescharacteristic of present-day earth, once again yields a mean glo-bal warming of only 0.4°C for a 300 ppm to 600 ppm increase inthe air’s CO2 content (Idso 1988). And no other simple line thatcan be drawn through these real-worlds data produces anygreater warming.

Summary and conclusionsEarth’s climate system possesses a number of highly effectivenegative feedback mechanisms that tend to inhibit CO2-inducedglobal warming. Some of these phenomena are driven by purelyphysical forces and they begin to exert their cooling influence inresponse to an initial rise in temperature. Others have a biolog-ical origin, but also respond to increasing warmth. The scientificliterature provides several demonstrations of their individual ca-pacities to negate totally the ultimate equilibrium warming thatis typically predicted to result from a doubling of the atmo-sphere’s carbon dioxide concentration.

To this arsenal of powerful climate-stabilizing forces can beadded yet a third set of real- world brakes on CO2-induced globalwarming: cooling forces that have their origins in biological phe-nomena that are directly enhanced by the aerial fertilization byincreased concentrations of atmospheric CO2. Operating with orwithout an initial impetus for warming, these forces have thepotential to lead to a cooling of the planet, since any of thewarming-induced negative feedbacks could nullify the green-house effect of a rise in atmospheric CO2, leaving the CO2-induced cooling forces to drive temperatures down even further.

Solid support for these feedback scenarios comes from a num-ber of real-world climatic observations. First and foremost arethe empirically based evaluations that I have made of the ulti-


mate warming likely to be produced by an increase in down-ward-directed thermal radiation equivalent to that expected tobe received at the surface of the earth as a result of a doubling ofthe air’s CO2 content, warming that is only one-tenth to one-third of what has typically been predicted for this situation bymost of the general-circulation models of the atmosphere thatare currently in vogue. Secondary support is provided by thesuite of recent observational studies that have revealed that con-temporary climate models have long significantly underestimat-ed the cooling power of clouds (Cess et al. 1995; Ramanathan etal. 1995; Pilewskie and Valero 1995; Heymsfield and McFarqu-har 1996), even when demonstrating the abilities of cloud-related cooling forces to negate totally the large global warmingthat is generally predicted to result from a doubling of the atmo-sphere’s CO2 concentration.

In view of these facts, I find no compelling reason to believethat the earth will necessarily experience any global warmingas a consequence of the ongoing rise in the atmosphere’s car-bon dioxide concentration. There could be a CO2-induced in-crease in mean global air temperature, but it would have to besmall—no more than 0.4°C for a 300 ppm to 600 ppm increasein the air’s CO2 content. Then, again, it is even possible thatthe planet could cool somewhat in response to a rise in atmo-spheric CO2. Our current understanding of the planet’s com-plex climate system is just not sufficient to draw any moredetailed conclusions.

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